Use of fluorinated hydrocarbons as reaction media for selective epoxidation of olefins

ABSTRACT

The present invention is directed to a process for the selective epoxidation of non-allylic olefins. The process includes the step of contacting a gas mixture comprising a non-allylic olefin, oxygen, and a fluorinated hydrocarbon with a silver epoxidation catalyst at conditions effective to epoxidize the non-allylic olefin. The fluorinated hydrocarbon has a C--F bond dissociation energy of 110 kcal/mole or greater, and sufficiently non-acidic C--H bonds, if present, so as to avoid abstraction of HF from the fluorinated hydrocarbon under reaction conditions.

FIELD OF THE INVENTION

This invention pertains to an improved gas phase process for theselective epoxidation on non-allylic olefins wherein the process iscarried out in the presence of certain fluorinated hydrocarbons. Thefluorinated hydrocarbons useful in the present invention have C--F bonddissociation energies high enough such that C--F bond rupture does notoccur at reaction temperature and have sufficiently non-acidic C--Hbonds, if any, so as to avoid abstraction of HF from the fluorinatedhydrocarbon.

BACKGROUND OF THE INVENTION

1. Flammability Limits

As explained in Lees, F. P., "Loss Prevention in the Process Industries,Volume 1," 485-86 (1980) and Coffee, R. D., Loss Prevention 13, 74-80,(1980), a flammable gas, e.g., methane, butane, ethylene, butadiene, andother hydrocarbons, burns in oxidizing environments only over a limitedcomposition range. The limits of flammability (often called theexplosive or hot flame limits) are the concentration extremes at which amixture of a flammable gas and an oxidant can continue to burn once aflame is ignited by an external energy source such as a spark. Theseflammability extremes are a function of temperature, pressure, andcomposition. The explosive limit is usually expressed as volume or molepercent of flammable gas in a mixture of oxidant (usually oxygen),inert, and flammable gas. The smaller value is the lower (lean) limitand the larger value is the upper (rich) limit. For example,methane-oxygen mixtures will propagate flames for methane concentrationsbetween 5.1 and 61 mole percent methane (i.e., 94.9 and 39 mole percentoxygen) and methane-air mixtures between 5.3 and 14 mole percent methane(i.e., 19.9 and 18 mole percent oxygen), at 25° C. and atmosphericpressure. In general, the lower explosive limit (LEL) decreases, and theupper explosive limit (UEL) increases as temperature and pressureincrease, and amount of inert decreases. Below a certain oxygen content,called the minimum oxygen content (MOC), the mixture will not supportcombustion. For methane at 25° C. and 1 atmosphere, the MOC is 13.98mole percent oxygen.

2. Diluents for Gas-Phase Epoxidation Reactions

When carrying out a highly exothermic reaction (e.g., epoxidation ofbutadiene or ethylene), it is important to design the reactor foradequate heat removal to prevent thermal runaway (uncontrollablereaction and generation of heat). A typical reactor configuration foroperation of a highly exothermic reaction is a multi-tubular packed bedimmersed in a flowing heat transfer fluid (often boiling water). In sucha reactor, heat is removed via several mechanisms: (1) axial convectionfrom the catalyst surface to the bulk gas feedstream; (2) radialconvection through the catalyst and support particles to the tube wallsand into the heat transfer fluid; and (3) radial conduction from thecatalyst surface through the bulk gas to the tube walls and into theheat transfer fluid. If the tubes are too large in diameter (radialtemperature gradient is large), the reactant or diluent gas has low heatcapacity, or the gas flow is too low, hot spots will develop which canlead to a runaway reaction or catalyst deactivation.

Processes for the selective epoxidation of olefins which contain noallylic hydrogen atoms (non-allylic olefins) or olefins which containhindered allylic hydrogen atoms are described by Monnier and Muehlbauerin U.S. Pat. Nos. 4,897,498; 4,950,773; 5,081,096; 5,138,077; and5,145,968. Stavinoha and Tolleson disclose in U.S. Pat. No. 5,117,012the selective epoxidation of 1,3-butadiene to 3,4-epoxy-1-butene (EpB)by contacting a mixture comprising 1,3-butadiene, oxygen, and methanewith a supported silver catalyst at elevated temperatures. Stavinoha etal. disclose in U.S. Pat. No. 5,362,890 an improved process for theselective epoxidation of 1,3-butadiene to EpB wherein the ballast gasfor the reaction is n-butane. Boeck et al. disclose in U.S. Pat. No.5,618,954 a similar process for the epoxidation of 1,3-butadiene to EpBwith nitrogen or C₁ -C₄ hydrocarbons as the diluent.

The use of diluent gases in non-allylic olefin epoxidation, specificallythe epoxidation of 1,3-butadiene to 3,4-epoxy-1-butene, is described inU.S. Pat. Nos. 5,362,890, and 5,618,954. U.S. Pat. No. 5,618,954discloses that nitrogen and C₁ -C₄ paraffinic hydrocarbons, especiallymethane, or mixtures thereof are the preferred diluents for theepoxidation of 1,3-butadiene to EpB. The oxygen:butadiene ratio in thereactor feed gas can be increased by using methane as the diluent overthat with nitrogen as the diluent without the methane:oxygen:butadienemixture becoming flammable.

U.S. Pat. No. 5,362,890 discloses the use of C₂ -C₆ paraffinhydrocarbons as diluents for non-allylic olefin epoxidation. The datadisclosed in this patent shows the advantages of using higher alkanehydrocarbons over methane, nitrogen, and other common diluents. Theadvantages cited include higher safe oxygen levels, higher epoxideproduction levels for the same reactor temperatures, and more stableoperation due to better heat removal.

The use of diluent or ballast gases in ethylene epoxidation is describedin Canadian Patent Nos. 1,286,687 and 2,053,404; and U.S. Pat. Nos.3,119,837 and 5,057,481. According to these patents, the typicalvolumetric composition of feed gases used in such ethylene epoxidationprocesses comprise 5 to 50 volume percent ethylene, 2 to 8 volumepercent oxygen, up to 7 volume percent carbon dioxide, up to 5 molepercent ethane with the balance being composed of an additional inertdiluent such as nitrogen or methane.

U.S. Pat. No. 3,119, 837 discloses that selectivity of ethyleneconversion to ethylene oxide can be enhanced by the addition of methaneas a diluent. Methane serves as a heat sink, moderating temperaturedifferentials within the reactor, and allows for more isothermal reactoroperation. This patent further states that the benefits to selectivityand ease of operation do not extend to other paraffins normallyencountered in commercially available ethylene, e.g., ethane andpropane, due to excessive stripping of chlorine from the surface of thecatalyst, which renders the catalyst unstable and, thus, susceptible tothermal runaway. Use of methane is also said to allow an increase in theoxygen:ethylene ratio in the reactor feed gas over the ratio withnitrogen, which increases conversion of ethylene to ethylene oxide.

According to Canadian Patent 1,286,687, other diluents that function asheat sinks include nitrogen, helium, argon, carbon dioxide, and lowerparaffins such as methane and ethane. However, U.S. Pat. No. 5,057,481discloses that the use of ethane at concentrations greater than about 5mole percent results in reduced selectivity in the epoxidation ofethylene to ethylene oxide and lower thermal stability because thechloride concentration on the catalyst surface is lowered. Typicalsilver catalysts employed in the epoxidation of ethylene contain about 1to 300 parts by million by weight (ppmw) of Cl on the catalyst surface,both to increase selectivity to ethylene oxide by lowering combustion ofethylene and ethylene oxide to carbon dioxide and water as well as toincrease the thermal stability of the silver catalyst. If the level ofCl on the surface of the silver catalyst becomes too low, the reactionbecomes excessively exothermic with accompanying loss of selectivity.Ethane acts as a chloride stripping agent and at concentrations above 5mole percent and at temperatures typically employed in the epoxidationof ethylene, e.g., 230 to 280° C., the degree of chloride strippingbecomes unacceptably excessive. As is disclosed in the above-citedpatents, one of the problems associated with the use of carbon dioxideas a heat transfer agent (heat sink) in ethylene epoxidation processesis that at levels greater than about 7 mole percent carbon dioxidebecomes a reaction inhibitor for ethylene oxide formation. Thus, theconcentration of carbon dioxide in feed gas in ethylene epoxidationprocesses must be limited to concentrations of less than about 7 molepercent. At such low levels, carbon dioxide does not have an appreciableeffect on the heat capacity nor the flammability characteristics of thereaction gas mixture.

Although much of the art discussed above has resulted in improvements inthe efficiency, activity, and/or stability of the epoxidation catalyst,there still exists a need in the art to further improve and increase theefficiency, activity, and stability of such catalysts. Accordingly, oneof the objects of the present invention is to provide a process thatmeets this need in the art.

SUMMARY OF THE INVENTION

Briefly, the present invention is directed to a process for theselective epoxidation of non-allylic olefins. The process includes thestep of contacting a gas mixture comprising a non-allylic olefin,oxygen, and a fluorinated hydrocarbon with a silver epoxidation catalystat conditions effective to epoxidize the non-allylic olefin. Thefluorinated hydrocarbon has a C--F bond dissociation energy of 110kcal/mole or greater, and sufficiently non-acidic C--H bonds, ifpresent, so as to avoid abstraction of HF from the fluorinatedhydrocarbon under reaction conditions.

DETAILED DESCRIPTION OF THE INVENTION

We have surprisingly discovered that certain fluorinated hydrocarbonscan be used as major components of gas streams for the selectiveepoxidation of olefins. These fluorinated hydrocarbons are used atconcentrations that would typically result in rapid and irreversibledeactivation of Ag-based catalysts through the formation of AgF, thethermodynamically stable end product. The main benefit of this discoveryis that feed compositions, e.g., the O₂ and C₄ H₆ concentrations forbutadiene epoxidation and the O₂ and C₂ H₄ concentrations for ethyleneepoxidation, can be increased such that the yield of the desired epoxideproduct is substantially increased, resulting in greater economicbenefit. This, of course, requires that the active and selective Agcatalysts are stable in the presence of such high concentrations offluorinated hydrocarbons. These fluorinated hydrocarbons are often usedas flame retardant and flame extinguishants, and have physical andchemical stability that make them especially appropriate as inertdiluents for feed streams that contact promoted Ag catalysts typicallyused for olefin epoxidation reactions.

It is noted that chlorinated hydrocarbons have also been used as flameretardant and flame extinguishants. However, the thermal and chemicalinstabilities of the chlorinated hydrocarbon compounds make themunsuitable for the purposes of this invention because total andirreversible loss of catalytic activity due to AgCl occurs when highconcentrations of chlorinated hydrocarbons are exposed to promotedAg-based catalysts.

Concentrations of inert, stable, fluorinated compounds in olefin and O₂feedstreams in accordance with the present invention are much higherthan the concentrations of organic chlorides typically added tofeedstreams of olefin epoxidation gas feedstreams. Parts per million(ppm) levels of organic halides, typically chlorides (see Monnier etal., U.S. Pat. No. 4,950,773 (1990) and D. J. Hucknall, "SelectiveOxidation of Hydrocarbons", Academic Press, London, 1974, pp. 6-19) arecontinually added to the feedstream to maintain selectivity and thermalstability during olefin epoxidation. Organic fluorides have also beenadded in ppm levels to the feedstreams of olefin epoxidation reactionsto maintain selectivity and thermal stability. Kapicek et al. in U.S.Pat. No. 4,994,588 (1991) describe the addition of the class offluorinated hydrocarbons known as "Freons" to the olefin and O₂containing feedstream. The concentration of fluorinated hydrocarbons ismaintained between 0.1 and about 2,000 ppm, by volume, of the overallfeedstream. For the successful use of fluorinated hydrocarbons in U.S.Pat. No. 4,994,588, it is also required that the fluorinatedhydrocarbons undergo partial decomposition during reaction. Thus, theuse of fluorinated hydrocarbons in U.S. Patent No. 4,994,588 fallsoutside the scope of the present invention.

Our discovery requires the addition of substantially higherconcentrations of fluorinated hydrocarbons that are stable underreaction conditions normally used for olefin epoxidation. Theconcentrations of the fluorinated hydrocarbon used in our invention aretypically between about 5 and about 70%, and more preferably, betweenabout 10 and about 60%, by volume of the feed gas mixture.

We have also discovered that the fluorinated hydrocarbon compoundswithin the scope of our invention have C--F bond dissociation energies(BDE) high enough such that C--F bond rupture does not occur at reactiontemperatures typically used for olefin epoxidation. BDE values in excessof 110 kcal/mole, and more preferably, in excess of 120 kcal/mole arerequired to ensure thermal stability under reactions, such thatformation of AgF does not occur.

In addition, the absence of reactive and acidic C--H bonds on thefluorinated hydrocarbon is required to ensure that abstraction of HFfrom the fluorinated hydrocarbon does not occur since reaction of HFwith Ag forms Ag--F, which is inactive for olefin epoxidation. Thelocation of the reactive C--H bond on fluorinated hydrocarbon is notmaterial, although it is known from U.S. Pat. No. 4,950,773 that C--Hbonds vicinal to, or adjacent, to C--X bonds, where X=Cl, Br, F, aremore reactive than C--H bonds which are geminal to C--F bonds (H-- andF-- bonded to the same C atom). For totally perfluorinated hydrocarbons,there are no remaining C--H bonds. In these cases, the only factor to beconsidered is the BDE of the various C--F bonds. Typically, BDE valuesare primary C--F>secondary C--F>tertiary C--F bonds.

Examples of fluorinated hydrocarbons useful in the process of ourinvention include without limitation CF₄, CHF₃, and C₂ F₆.Hexafluoroethane is especially preferred, since it contains only primaryC--F bonds with a BDE of 126.8 kcal/mole and has a high molar heatcapacity, favorable for heat management and flammability control in theepoxidation reactor.

Catalyst and Catalyst Preparation

The supported silver epoxidation catalysts that may be used in theprocess of our invention are known materials which may be preparedaccording to published procedures. Various alkali salt and thallium(I)salt promoters, various Ag salts used as Ag catalyst precursors, andvarious catalyst carriers used to support the promoted Ag catalysts havebeen discussed in earlier patents. Thermal and chemical methodologiesused to generate active and selective promoted silver catalysts havealso been described. See, for example, the following patents: J. L.Stavinoha et al., U.S. Pat. No. 5,362,890 (1994); J. R. Monnier et al.,U.S. Pat. No. 5,138,077 (1992); J. R. Monnier et al., U.S. Pat. No.4,950,773 (1990); J. E. Buffum et al., U.S. Pat. No. 5,145,824 (1992);M. M. Bhasin et al., U.S. Pat. No. 4,916,243 (1990); and A. M.Lauritzen, U.S. Pat. No. 4,833,261 (1989).

These references describe in great detail and in name different promotersalts and ranges of promoter salt loadings that have been used topromote both ethylene and butadiene and related olefins for selectiveepoxidation. Likewise, the above patents describe in great detail and inname specific Ag salts and ranges of silver salt precursors that havebeen used to generate promoted silver catalysts for both ethylene andbutadiene and related olefins for selective epoxidation. Moreover, theabove references specify in great detail and in name inert carriers thathave been used to support the above catalyst components to carry out theepoxidation of ethylene, butadiene, and related olefins for selectiveepoxidation. Finally, the above patents discuss in great detailsynthetic methodologies used to generate active and selective promoted,silver catalysts for ethylene, butadiene, and related olefinepoxidation. Familiarity with the content of those patents are presumedand are therefore not recited herein. The contents of which, however,are all hereby incorporated by reference.

The preferred epoxidation catalyst for use in the present invention is aCs-promoted, supported Ag catalyst. Such a catalyst has been describedin the literature and can be prepared using methods currently employedin the art.

Olefin Reactants and Reaction Conditions

The olefin reactants that may be used in the process of our inventioninclude norbornene, norbornadiene and olefins having the general formula##STR1## wherein R¹ is hydrogen or alkyl and R² is hydrogen, an arylgroup, a tertiary alkyl group such as tertiary butyl, tertiary amyl, ortertiary octyl, or the group having the formula ##STR2## with theproviso that R¹ contains no hydrogen atoms in a position allylic to theethylenic unsaturation, i.e., the >C═C< group or groups. The alkylgroups represented by R¹ may be unsubstituted or substituted alkylhaving up to about 12 carbon atoms. Such alkyl groups preferably areunsubstituted alkyl of up to about 4 carbon atoms.

When the reactant is an olefin having the formula ##STR3## the R¹substituents may be the same or different.

The aryl groups represented by R² may be unsubstituted or substitutedcarbocyclic aryl having 6 to 10 carbon atoms, e.g., unsubstituted andsubstituted phenyl and naphthyl radicals. Examples of the substituentswhich may be present on the aryl groups include alkyl of up to about 4carbon atoms, alkoxy of up to about 4 carbon atoms, halogen such aschloro and bromo, hydroxy, vinyl, and the like so long as none of thesubstituents have a hydrogen allylic to a double bond.

The epoxides produced from the olefins of formula (I) in accordance withthe epoxidation process described herein have the general formula##STR4## wherein R¹ and R² are defined above. The process provided byour invention is especially useful for the selective monoepoxidation ofethylene to ethylene oxide and 1,3-butadiene to 3,4-epoxy-1-butene.

Our novel process may be carried out at a temperature in the range ofabout 100° to 400° C., with the range of 150° to 300° C. beingparticularly preferred. The pressure within the epoxidation zone mayrange from about 0.5 to about 30 bars, preferably from about 1 to about20 bars. It is apparent that the particular combination of temperatureand pressure is selected so as to maintain all of the components of thefeed to the epoxidation zone in the gaseous state.

As mentioned above, the advantages of the present invention may beachieved by feeding to the epoxidation zone a gas mixture comprisingfrom about 5 to about 70% by volume, and more preferably, from about 10to about 60% by volume of the fluorinated hydrocarbon. Normally, thefeed gas mixture will also contain from about 5 to about 50% by volumeof the olefin reactant, from about 3 to about 40% by volume of oxygen,and 0 to about 80% by volume of other components such as paraffinhydrocarbon containing 1 to 6 carbon atoms, organic halide, water,nitrogen, carbon dioxide, argon, helium, and recycled epoxide product.

To further illustrate the present invention and the advantages thereof,the following specific examples are provided. It is to be understoodthat these examples are merely intended to be illustrative and notlimitative.

EXAMPLES Catalyst Evaluations

For all catalyst evaluations, catalyst samples were ground and sieved to18/25 mesh (between 0.71 and 1.00 mm in diameter) and loaded intoreactors. Reactor dimensions were 0.375" OD stainless steel reactor withan inside diameter of 0.305". The reactor had an overall length of 23.7inches and the catalyst was maintained inside the tubular reactor on astainless steel mesh screen that supported the catalyst charge ofapproximately 12.0 grams in the middle of the reactor. A thermowell wasinserted in the middle of the catalyst bed. The catalyst bed temperaturewas determined by moving a smaller ID thermocouple up and down insidethe thermowell to determine the hot spot in the 12" long catalyst bed.The temperatures reported in the examples are the hot spot in the bed.Surrounding all but the ends of the tubular reactor was a 1.0" OD brassjacket into which the 0.375" tube had been press fitted. The 1" OD brassjacket helped to maintain isothermal conditions throughout the length ofthe catalyst bed by acting as an efficient heat sink. Pressure wasmaintained at the desired level by using either a needle valve torestrict flow or by a spring loaded, back pressure regulator. Below thepressure letdown, the gas flow was at approximately 14.7 psia. The gasflow was directed through an in-line gas-sampling network that permittedreal time, accurate analysis of catalyst performance. Contents of a gassample loop were analyzed using a Hewlett-Packard gas chromatographemploying a Poraplot Q column hooked to a thermal conductivity detector.This detector gave quantitative analyses for oxygen, carbon dioxide,water, butadiene, 3,4-epoxy-1-butene, and n-butane. Further, by means ofa switching valve, a diverted slipstream of the feed gas above thecatalyst could be analyzed to determine the feed gas composition;comparison with the analysis of the product stream permitted calculationof the conversion of butadiene feed and selectivity to epoxybutene. Asused herein, conversion is the mole percent conversion of butadienedefined as: ##EQU1## and selectivity is the percent selectivity to3,4-epoxy-1-butene defined as: ##EQU2## High Pressure Evaluation

For high-pressure operation, gas delivery was accomplished using massflow controllers for oxygen, propane, ppm levels of 2-chlorobutane, andhexafluoroethane. Butadiene was added as a liquid by means of a dualsyringe-pump assembly, which permitted continuous delivery of butadieneat high-pressure conditions. The liquid butadiene was flash vaporizedinto the gas feed line in a high temperature, liquid vaporizermaintained at 100° C. The blended gas phase composition was preheatedand passed over the catalyst and performance subsequently determined.

Before exposure to reaction gas feeds, each catalyst was pretreated at250° C. in a feedstream containing 20% oxygen in 80% helium forapproximately 2 hrs, followed by exposure to a gas stream containing 10ppm of 2-chlorobutane. The catalyst was then evaluated for epoxybuteneformation.

Evaluation at Atmospheric Pressure

The same type of stainless steel reactors with brass jackets was usedfor the one-atmosphere evaluation experiments. In-line analyses wereconducted using similar gas sampling methods and gas chromatographicanalysis. Conversions of butadiene and selectivity to epoxybutene aredefined as above. All feed compositions, with the exception of CCl₄,were added to the feedstream using mass flow controllers. Specifically,oxygen, butadiene, n-butane, ppm levels of 2-chlorobutane,trifluoromethane (HCF₃), hexafluoroethane (C₂ F₆),1,1,2,2-tetrafluoroethane (HFC-134a), octafluoropropane (C₃ F₈), andn-decafluorobutane (n-C₄ F₁₀) were added to the feedstream using massflow controllers. Carbon tetrachloride (CCl₄) was supplied to thefeedstream from a vapor liquid equilibrium saturator, whereby an inertsweep gas, typically helium, was passed through liquid CCl₄, maintainedat a predetermined temperature. By proper combination of saturatortemperature and sweep gas flow, the desired concentration of CCl₄ wasadded to the feedstream contacting the catalyst.

Before evaluation, all catalysts were pretreated by calcining in aflowing stream of 20% oxygen and 80% helium for 2 hours at 250° C.,followed by pretreatment in 20% hydrogen and 80% helium for one hour at200° C. Catalysts were then ready to be evaluated for activity andselectivity to epoxybutene.

Comparative Example 1

3.0 grams of a catalyst containing 700 ppm of Cs as a promoter and 15%Ag supported on a fused alpha alumina carrier were pretreated andevaluated at one atmosphere pressure as described above. The catalystwas first exposed at 190° C. and 200° C. for a total of 23 hrs to a feedgas made up of 67% n-C₄ H₁₀, 16.5% C₄ H₆, 16.5% O₂, and 2 ppm2-chlorobutane. After 23.0 hrs, the feed gas was switched to 33% n-C₄H₁₀, 14% He, 20% CCl₄, 16.5% C₄ H₆, 16.5% O₂, and 2 ppm 2-chlorobutane.Catalyst performance at 190° C. was measured 3 min and 20 min after CCl₄addition and at 195° C. after exposure to CCl₄ for a total of 35 min.The results are summarized in Table 1.

                  TABLE 1                                                         ______________________________________                                        Effect of CCl.sub.4 on Performance                                                                          C.sub.4 H.sub.6                                                                        EpB                                      Temp. Run Time CCl.sub.4 in Feed Conversion Selectivity                       (°C.) (hrs) (vol %) (%) (%)                                          ______________________________________                                        190    20.50      0         5.33     91.10                                      195 20.80  0 6.69 90.85                                                       190 23.00  0 5.23 91.01                                                     20% CCl.sub.4 added to feed after 23.30 hrs on-line.                          ______________________________________                                          190      23.35     20       0.64     12.47                                    190 23.63 20 0.05  0.00                                                       195 23.88 20 0.01  0.00                                                     ______________________________________                                    

The results show that exposure to 20% CCl₄ for periods as short as 0.05hrs results in massive loss of activity. After only 0.58 hrs of totalexposure to 20% CCl₄, total loss of activity occurs. Analysis of thecatalyst after reaction by x-ray diffraction indicated that thecatalytically active Ag surface had been completely and irreversiblyconverted to AgCl, which is inactive for olefin epoxidation. The bonddissociation energy (BDE) for the C--Cl bond in CCl₄ has been reportedto be 73.1 kcal/mole ("Handbook of Chemistry and Physics, 73^(rd)Edition," D. R. Lide, editor-in-chief, CRC Press, Boca Raton, Fla.,1992, pp. 9-138 to 9-141).

Comparative Examples 2-4

Catalyst samples containing 700 ppm Cs, 15% Ag supported on fused alphaalumina were ground and sieved to 18/15 mesh. Approximately 12.0 gms ofsieved catalyst were used for each experiment. Fresh catalyst chargeswere used for each experiment. Each catalyst was pretreated asreferenced in Comparative Example 1 before evaluation. The catalystswere allowed to reach steady-state activity at 200° C. in a feedcomposition of 67% n-C₄ H₁₀, 16.5% C₄ H₆, 16.5% O₂, and 2 ppm2-chlorobutane before the addition of 20% fluorinated hydrocarbon to thefeedstream. Catalysts were typically evaluated for a period of 20-24 hrsin the above feedstream before steady-state performance conditions wereattained. After steady-state reaction rates were attained, the feedcomposition was changed to 47% n-C₄ H₁₀, 20% fluorinated hydrocarbon,16.5% C₄ H₆, 16.5% O₂, and 2 ppm 2-chlorobutane. In all cases, totalflow rate was maintained at 300 ml (STP)/min. Catalytic activity (C₄ H₆conversion) and selectivity to epoxybutene (EpB) were followed as afunction of exposure time to the feed containing 20% fluorinatedhydrocarbon. The results are summarized below in Table 2 for catalystperformance using 20% C₃ F₈ (Comparative Example 2), 20% n-C₄ F₁₀(Comparative Example 3), and 20% HFC-134a (Comparative Example 4) in thefeed gas to the catalyst. Catalyst performance before addition of thefluorinated hydrocarbon is noted as 0 hrs of addition.

                  TABLE 2                                                         ______________________________________                                        Effect of Certain Fluorinated Hydrocarbons on Catalyst Performance              Com-     Fluo-    Length of                                                                                            parative rinated Time Conversio                                              n Selectivity C-F BDE                 Example Hydro- Added of C.sub.4 H.sub.6 to EpB (kcal/                         No. carbon (hrs) (%) (%) mole)                                              ______________________________________                                        2      C.sub.3 F.sub.8                                                                        0        26.7    86.2   110                                       2 12.8 92.2 (secondary                                                           C-F)                                                                       3 9.5 92.1 123                                                                7 2.7 92.6 (primary                                                           17 0.9 75.3 C-F)                                                            3 n-C.sub.4 F.sub.10 0 22.5 88.1 110                                            2 0 0 (secondary                                                                 C-F)                                                                       4 0 0 123                                                                     8 0 0 (primary                                                                   C-F)                                                                     4 HFC-134a 0 27.2 85.5 C-F bond:                                                3 25.7 86.3 120                                                               23 19.2 91 C-H bond:                                                          37 7.7 93.1 95                                                                55 3.5 90.9                                                               ______________________________________                                    

The rapid and irreversible loss of catalytic activity is due to thereactivity of abstracted F atoms with the Ag surface of the Cs-promoted,Ag/Al₂ O₃ catalyst. In the case of HFC-134a, the BDE of the C--H bondcontributes to facile formation of H--F. The formation of AgF rendersthe Ag catalyst inactive for butadiene epoxidation.

Example 1

12.1 grams of a catalyst containing 700 ppm of Cs as a promoter and 15%Ag supported on a fused alpha alumina carrier were pretreated andevaluated at one atmosphere pressure as described above. The catalystwas first exposed at 200° C. for a total of 19 hrs to a feed gas made upof 67% n-C₄ H₁₀, 16.5% C₄ H₆, 16.5% O₂ and 2 ppm 2-chlorobutane. After19 hrs, the feed composition was changed to 47% n-C₄ H₁₀, 20% C₂ F₆,16.5% C₄ H₆, 16.5% O₂, and 2 ppm 2-chlorobutane. For both feedcompositions, total flow rate was maintained at 300 ml (STP)/min.Catalytic activity (C₄ H₆ conversion) and selectivity to epoxybutene(EpB) were followed as a function of exposure time to the feedcontaining 20% C₂ F₆. Catalyst performance is summarized in Table 3below.

                  TABLE 3                                                         ______________________________________                                        Effect of 20% C.sub.2 F.sub.6 on Catalyst Performance                           Length of Time                                                                C.sub.2 F.sub.6 Added Conversion of C.sub.4 H.sub.6 Selectivity to EpB                                     (hrs) (%) (%)                                  ______________________________________                                        0            22.2         89.3                                                  2 22.5 88.7                                                                   5 22.4 89.3                                                                   9 22.4 89.1                                                                   21 21.6 88.9                                                                  35 20.8 88.9                                                                  49 20.5 89.5                                                                  69 20.3 89.5                                                                  105 20.5 89.2                                                                 139 20.3 89.3                                                               ______________________________________                                    

The bond dissociation energy (BDE) of the C--F bonds in C₂ F₆ is 127kcal/mole (Bryant, W. M. D., J. Polymer Science, vol. 56, pages 277-296,1962). This bond energy is considerably higher than bond dissociationenergies of C--F bonds in Comparative Examples 2-4 and is much largerthan the C--Cl bond dissocaiation energy in Comparative Example 1. Theresistance of C₂ F₆ to F abstraction by the promoted Ag catalyst to formAgF results in stability of the Cs-promoted, Ag catalyst, even atconcentrations as high as 20% in the feedstream and at temperatures ashigh as 200° C. The bond dissociation energy of the C--F bond inHFC-134a is comparable to that for C₂ F₆. However, the presence ofacidic C--H bonds in HFC-134a favor F abstraction by dehydrofluorinationto form HF, which results in catalyst deactivation the same way that Fabstraction from C₃ F₈ and n-C₄ F₁₀ results in catalyst deactivation.Catalyst modification by dehydrohalogenation is disclosed in U.S. Pat.No. 4,950,773 (August 1990).

Example 2

12.0 grams of a catalyst containing 700 ppm of Cs as a promoter and 15%Ag supported on a fused alpha alumina carrier were pretreated andevaluated at one atmosphere pressure as described above. The catalystwas first exposed at 200° C. for a total of 48 hrs to a feed gas made upof 67% n-C₄ H₁₀, 16.5% C₄ H₆, 16.5% O₂, and 2 ppm 2-chlorobutane. After48 hrs, the feed composition was changed to 47% n-C₄ H₁₀, 20% HCF₃,16.5% C₄ H₆, 16.5% O₂, and 2 ppm 2-chlorobutane. For both feedcompositions, total flow rate was maintained at 300 ml (STP)/min.Catalytic activity (C₄ H₆ conversion) and selectivity to epoxybutene(EpB) were followed as a function of exposure time to the feedcontaining 20% HCF₃. Catalyst performance is summarized in Table 4below.

                  TABLE 4                                                         ______________________________________                                        Effect of 20% HCF.sub.3 on Catalyst Performance                                 Length of Time                                                                HCF.sub.3 Added Conversion of C.sub.4 H.sub.6 Selectivity to EpB                                           (hrs) (%) (%)                                  ______________________________________                                        0            12.2         92.0                                                  2 12.8 92.2                                                                   10 13.0 92.7                                                                  20 13.0 93.3                                                                  30 13.5 93.5                                                                  50 13.8 93.5                                                                  70 13.2 93.8                                                                ______________________________________                                    

The bond dissociation energy of the C--F bond in HCF₃ is 128 kcal/mole("Handbook of Chemistry and Physics, 73^(rd) Edition," D. R. Lide,editor-in-chief, CRC Press, Boca Raton, Fla., 1992, pp. 9-138 to 9-141).This high bond dissociation energy results in excellent stability ofHCF₃ in the presence of the Cs-promoted, Ag catalyst. Thus, nodeactivation occurs, even in the presence of 20% HCF₃ in the feedstream.While HCF₃ does contain a C--H bond, both H and F are attached to thesame C atom. This results in much greater stability and resistance todehydrofluorination to form HF, which is markedly different from thecase of HFC-134a, where the reactive H and F are on adjacent C atoms.

Example 3

12.0 grams of a Cs-promoted, Ag catalyst promoted on fused alpha aluminacontaining 1310 ppm Cs and 15.8% Ag were evaluated at high-pressureconditions using a reactor and analytical system as described above. Thecatalyst was pretreated at 250° C. for 2 hrs in 20% O₂, balance inertgas and an additional 30 minutes in a gas stream containing 10 ppm2-chlorobutane at 180° C. The catalyst was then exposed to a gas streamcontaining 9% C₄ H₆, 17% O₂, balance C₃ H₈ +2 ppm of 2-chlorobutane at200° C. and 15 psia total pressure. After 50 hours, the overall pressurewas increased to 45 psia total pressure and the temperature raised to205° C. The total gas flow rate was maintained constant at 450 ml(STP)/min.

After approximately 150 hours on-line, the reactor pressure wasincreased to 60 psia. Temperature and total flow were held constant at205° C. and 450 ml (STP)/min, respectively. Gas feed compositions werechanged as summarized in Table 5 to, at times, include C₂ F₆ as a feedcomponent; when C₂ F₆ was added, the C₄ H₆ and O₂ concentrations werealso increased. The overall gas compositions were selected to ensurethat operation remained in the non-flammable region, based ontemperature and pressure. Of course, addition of C₂ F₆ permitted safe,non-flammable gas compositions with higher C₄ H₆ and O₂ concentrations,illustrating an especially important benefit of using C₂ F₆ as thepreferred inert diluent.

                  TABLE 5                                                         ______________________________________                                        Effect of 20% C.sub.2 F.sub.6 on Catalyst Performance at 60 psia Pressure         Feed Concentration (%) at 60 psia                                                                 Butadiene EpB                                           Pressure Conversion Selectivity                                             C.sub.4 H.sub.6                                                                      O.sub.2 C.sub.3 H.sub.8                                                                      C.sub.2 F.sub.6                                                                       (%)     (%)                                     ______________________________________                                        9      22      69     0       13.7    85.6                                      9 26 45 20 16.5 86.9                                                          15 21 64 0 15.6 86.7                                                          15 25 40 20 17.4 87.6                                                       ______________________________________                                    

The above data illustrate very clearly the benefits of replacing some ofthe propane diluent with C₂ F₆. Replacement of 20% C₃ H₈ with C₂ F₆permits non-flammable and non-explosive operation with higherconcentrations of O₂ at two different levels of C₄ H₆ concentration. At9% C₄ H₆ concentration, the presence of C₂ F₆ allows O₂ to be increasedfrom 22 to 26%; while at 15% C₄ H₆, O₂ levels can be raised from 21 to25%. In both cases, conversion and selectivity increased, resulting inhigher yields of epoxybutene.

Example 4

12.0 grams of a catalyst containing 700 ppm of Cs as a promoter and 15%Ag supported on a fused alpha alumina carrier were pretreated andevaluated at one atmosphere pressure as described above. The catalystwas first exposed at 190° C. for a total of 140 hrs to a feed gas madeup of 70% n-C₄ H₁₀, 12% C₄ H₆, 18% O₂, and 2 ppm 2-chlorobutane. After140 hrs, the feed composition was changed to 50% C₂ F₆, 20% C₄ H₆, 30%O₂, and 2 ppm 2-chlorobutane for a period of 24 hrs. For both feedcompositions, total flow rate was maintained at 155 ml (STP)/min. Thefeed composition of 70% n-C₄ H₁₀, 12% C₄ H₆, 18% O₂, and 2 ppm2-chlorobutane was selected to give the most reactive, yet non-flammableand non-explosive, feed composition attainable at the described reactionconditions. This feed composition maximizes the activity for EpBformation. The feed composition containing 50% C₂ F₆ is non-flammableand non-explosive at any reaction or process condition. Any combinationof C₄ H₆ and O₂ totalling 50% of the total gas feed could be safely usedboth in the reactor and downstream processing below the reactor. Thiscomposition was used for 24 hours with no decrease in catalystperformance. The reaction temperature was then raised to 200° C. and thecatalyst exposed again to a feed containing 70% n-C₄ H₁₀, 12% C₄ H₆, 18%O₂, and 2 ppm 2-chlorobutane and then to 50% C₂ F₆, 20% C₄ H₆, 30% O₂,and 2 ppm 2-chlorobutane. The results are summarized below in Table 6.Percent EpB produced refers to concentration of EpB in the productstream, expressed in volume percent of the total gas stream. It isnecessary to express catalyst activity in this manner, since the C₄ H₆composition varies from 12% to 20% in the examples in Table 6.

                  TABLE 6                                                         ______________________________________                                        Effect of 50% C.sub.2 F.sub.6 on Catalyst Performance                           Reaction                    EpB     Selectivity                               Temperature Feed Composition (%) Produced to EpB                            (° C.)                                                                         n-C.sub.4 H.sub.10                                                                     C.sub.2 F.sub.6                                                                      C.sub.4 H.sub.6                                                                     O.sub.2                                                                           (%)     (%)                                 ______________________________________                                        190     70       0      12    18  0.80    92.8                                  190 0 50 20 30 1.44 91.2                                                      200 70 0 12 18 1.47 91.7                                                      200 0 50 20 30 1.79 92.2                                                    ______________________________________                                    

The data in Table 6 indicate that enhanced, stable catalyst performanceoccurs when feed levels contain as high as 50% C₂ F₆. Further, thepresence of 50% C₂ F₆ in the feedstream renders the entire butadieneepoxidation process non-flammable and non-explosive. Operation of olefinepoxidation processes in safe non-flammable and non-explosiveoperational regimes is a very important consideration in operation ofcommercial, large-scale olefin epoxidation processes. Secondly, the datain Table 6 show that catalyst deactivation does not occur forfeedstreams containing stable fluorohydrocarbons at the very high levelsnecessary to ensure non-flammable and non-explosive operation. The safeoperability regime permits any desirable combination of olefin and O₂totaling 50% of the total feed composition. Of course, it is understoodthat organic chloride levels, in this case, 2-chlorobutane, becontinually added to the feedstream in ppm levels.

Comparative Example 5 and Example 5

Computer simulations of a commercial reactor tube were conducted usingkinetic parameters fitted for the epoxidation of ethylene to ethyleneoxide with a commercial silver catalyst. A two-dimensional heat and masstransfer model of the epoxidation reaction (taking into accounttransport in both axial and radial dimensions) was found to be inexcellent agreement with the experimentally measured conversions,selectivities, and temperature profiles. The computer simulation wasused to determine the expected selectivity, conversion, and temperatureprofile for the conversion of ethylene to ethylene oxide with eithermethane or perfluoroethane as the diluent or ballast gas. Eachsimulation run was conducted at 2 percentage points below the maximumsafe oxygen concentration for the given system at a nominal maximumtemperature of 250° C., outlet pressure of 1.17 MPa, constant reactorvolume of 20.8 cubic meters, and constant centerline temperature at thehot spot. A summary of feed concentrations and simulation results isgiven in Table 7 below for a production rate of 100 million pounds peryear of ethylene oxide. The maximum safe oxygen concentation isincreased by 5 percentage points by the replacement of methane with C₂F₆.

                  TABLE 7                                                         ______________________________________                                        Effect of Diluent on Reactor Performance                                                      Comparative                                                     Example 5 Example 5                                                         ______________________________________                                        Inlet Conditions                                                                Reactor Length (m) 10.7 7.6                                                   Inside Tube Diameter (m) 0.041 0.041                                          Number of Tubes 1486 2080                                                     Reactor Volume (m.sup.3) 20.8 20.8                                            Ethylene Feed Concentration 35 mole % 35 mole %                               Diluent Concentration 38.0 mole % CH.sub.4 33.6 mole % C.sub.2 F.sub.6                                   Maximum Safe Oxygen Level 11.0 mole % 16.0                                   mole %                                              Oxygen Level Used in 9.0 mole % 14.0 mole %                                   Simulation                                                                    Outlet Conditions                                                             Outlet Pressure (MPa) 1.17 1.17                                               Centerline Temperature at Hot 255 255                                         Spot (°C.)                                                             Average Radial Temperature at 251 253                                         Hot Spot (°C.)                                                         Radial Temperature Differential 8.5 4.6                                       at Hot Spot (°C.)                                                      Ethylene Conversion (%) 10.4 11.9                                             Selectivity to Ethylene Oxide 75.5 77.4                                       (%)                                                                           Space-Time Yield (lb EO/lb 11.0 18.2                                          catalyst-hr)                                                                  Outlet Ethylene Oxide 2.77 mole % 3.26 mole %                                 Concentration                                                               ______________________________________                                    

The simulations clearly show the advantage of replacing the methanediluent with C₂ F₆. The reactor is made more isothermal (radial andaxial temperature gradients decreased), a higher EO concentration isseen at the reactor outlet with a shorter bed, and the space-time yieldis increased with C₂ F₆. At equivalent reactor volumes, the productionrate and space time yield are increased by 64% by replacing methane withC₂ F₆.

As seen from the above, addition or replacement of diluent hydrocarbonsin epoxidation feed gases with certain fluorinated hydrocarbon compoundsin accordance with our invention provides a number of advantages. Thoseadvantages include:

1. Increasing the maximum safe oxygen concentration. This allows the useof higher O₂ levels than possible with hydrocarbon diluents, whilemaintaining operation outside of the flammability envelope. An increasein O₂ partial pressure increases the rate of epoxidationreactions--resulting in a higher space-time-yield.

2. Rendering the reactor more isothermal. The fluorinated hydrocarboncompounds suitable for use in the present invention have high heatcapacities, much higher than methane, nitrogen, and other commondiluents. Since epoxidations are highly exothermic and prone to thermalrunaways, the addition of those fluorinated hydrocarbons increases theheat capacity of the gas in the reactor, making the reactor moreisothermal. A more isothermal reactor is easier to control and safer tooperate.

3. Improving safety. Flammable hydrocarbons are replaced bynon-flammable fluorinated hydrocarbon compounds. The inventory offlammable gas in the reactor and recycle loop is thus reduced. Lessflammable gas would be released in an accident, and the operation of theplant is made safer by this change.

While the invention has been described with reference to workingexamples and preferred embodiments, it is to be understood thatvariations and modifications may be resorted to as will be apparent tothose skilled in the art. Such variations and modifications are to beconsidered within the purview and scope of the invention as defined bythe claims appended hereto.

We claim:
 1. A process for the selective epoxidation of non-allylicolefins, said process comprising the step of:contacting a gas mixturecomprising a non-allylic olefin, oxygen, and a fluorinated hydrocarbonwith a silver epoxidation catalyst at conditions effective to epoxidizethe non-allylic olefin, wherein said fluorinated hydrocarbon has a C--Fbond dissociation energy of 110 kcal/mole or greater, and sufficientlynon-acidic C--H bonds, if present, so as to avoid abstraction of HF fromthe fluorinated hydrocarbon under reaction conditions.
 2. The processaccording to claim 1, wherein said non-allylic olefin is ethylene or1,3-butadiene.
 3. The process according to claim 1, wherein saidfluorinated hydrocarbon has a C--F bond dissociation energy of 120kcal/mole or greater.
 4. The process according to claim 1, wherein saidfluorinated hydrocarbon is CF₄, C₂ F₆, CHF₃, or mixtures thereof.
 5. Theprocess according to claim 1, wherein said gas mixture comprises fromabout 5 to about 70 vol % of said fluorinated hydrocarbon.
 6. Theprocess according to claim 5, wherein said gas mixture comprises fromabout 10 to about 60 vol % of said fluorinated hydrocarbon.
 7. A processfor the selective epoxidation of ethylene, said process comprising thestep of:contacting a gas mixture comprising ethylene, oxygen, and fromabout 5 to about 70 vol % of a fluorinated hydrocarbon with a silverepoxidation catalyst at conditions effective to epoxidize thenon-allylic olefin, wherein said fluorinated hydrocarbon has a C--F bonddissociation energy of 120 kcal/mole or greater, and sufficientlynon-acidic C--H bonds, if present, so as to avoid abstraction of HF fromthe fluorinated hydrocarbon under reaction conditions.
 8. The processaccording to claim 7, wherein said fluorinated hydrocarbon is CF₄, C₂F₆, CHF₃, or mixtures thereof.
 9. The process according to claim 7,wherein said gas mixture comprises from about 10 to about 60 vol % ofsaid fluorinated hydrocarbon.
 10. A process for the selectiveepoxidation of 1,3-butadiene, said process comprising the stepof:contacting a gas mixture comprising 1,3-butadiene, oxygen, and fromabout 5 to about 70 vol % of a fluorinated hydrocarbon with a silverepoxidation catalyst at conditions effective to epoxidize thenon-allylic olefin, wherein said fluorinated hydrocarbon has a C--F bonddissociation energy of 120 kcal/mole or greater, and sufficientlynon-acidic C--H bonds, if present, so as to avoid abstraction of HF fromthe fluorinated hydrocarbon under reaction conditions.
 11. The processaccording to claim 10, wherein said fluorinated hydrocarbon is CF₄, C₂F₆, CHF₃, or mixtures thereof.
 12. The process according to claim 10,wherein said gas mixture comprises from about 10 to about 60 vol % ofsaid fluorinated hydrocarbon.